FIG. 1 illustrates a prior art sealess pump having process lubricated journal bearing made from ceramic materials, such as, silicon carbide. The pump has an impeller (not illustrated) which is located to the right of drive shaft 12. To the left of drive shaft 12 is located a motor drive (not illustrated). A stationary ceramic bushing 14 is mounted within the housing 16 of the pump. An inner bore 18 of the stationary ceramic bushing 14 forms a rotational support for a assembly 20 which is fixed to shaft 12 by a preload force F which is applied by a bolt assembly (not illustrated). The inner surface 18 of the stationary bushing 14 and the outside surface 22 of the assembly 20 form the rotatable support for the shaft 12 of the pump which is lubricated by fluid during operation.
The assembly 20 is comprised of a cylindrical ceramic sleeve 24 which is held against rotation with respect to the shaft 12 by a resilient gasket or wave spring 26 located outboard of axial ends of the cylindrical sleeve 24. Steel end caps 28 transfer the preload force F to the ceramic sleeve 24. The residual compression provided by the preload force F provides the necessary axial tightness in the assembly to prevent rotation of sleeve 24 relative to the shaft 12. Preload F at assembly must be kept within a narrow range to insure proper tightness at maximum operating temperatures to prevent rotation. With a pump or other rotary apparatus which is subject to thermal cycling, incorrect preloading of the assembly can result in catastrophic failure of the outer surface of the shaft 12 that faces the inner surface of the ceramic sleeve 24 as well as the inner surface 18 of the stationary bushing 14 and the outer surface 22 of the cylindrical sleeve 24.
While the prior art of FIG. 1 has been used successfully, it is a complicated structure requiring careful assembly to maintain the preload F within the proper narrow range to avoid spinning of the assembly 20 with respect to the shaft 12 throughout a wide range of operational temperatures.
FIG. 2 illustrates a prior art static assembly for maintaining a preload on a first stationary element 30 and a thermal compensating sleeve 32. The preload is established by applying torque to a fastener 34 which is threaded into a bore 36 of a second stationary element 38. The second stationary element 38 may be a pressure vessel with a seal 40 being located between the first and second stationary elements for retaining a pressurized fluid. It should be understood that only a portion of the first and second stationary elements 30 and 38 have been illustrated for the purpose of explaining the prior art. The first stationary element 30 has a thermal coefficient of expansion less than the thermal coefficient of expansion of fastener 34. The thermal compensating sleeve 32 is manufactured from a material having a thermal expansion chosen to provide thermal compensation for a particular temperature range of operation. Thermal compensation provided by the thermal compensating sleeve 32 is based upon the elongation of the fastener 34. The extended length of the fastener 34 resultant from the thermal compensating sleeve 32, the thermal coefficients of expansion of the first stationary member 30, thermal compensation sleeve 32 and fastener 34 and the temperature range of operation determine the final preload characteristic throughout the temperature range of operation. However, the thickness of the first stationary member 30 located between a bottom surface 44 of the thermal compensating sleeve 32 and a top surface 46 of the second stationary element 38 is sufficiently great that substantial differential expansion occurs between the first stationary element 30 and the fastener 34. As a result, maintaining a specified preload during thermal cycling of the assembly of FIG. 2 may be difficult when the temperature range of the thermal cycling is great.